During neurogenesis in Drosophila, groups of ectodermal cells are endowed with the capacity to develop into neuronal precursors. The Notch signaling pathway is required to limit the neuronal potential to one or two cells within each group. Loss of genes of the Notch signaling pathway results in a neurogenic phenotype: hyperplasia of the nervous system accompanied by a parallel loss of epidermis.
Echinoid (Ed), a cell membrane associated Immunoglobulin C2-type protein has previously been shown to be a negative regulator of the EGFR pathway during eye development. Work from our lab has shown that Ed has a role in restricting neurogenic potential during embryonic neurogenesis. I have extended the functional and genetic analysis of Ed. Loss of ed suppresses the lack of neuronal elements caused by ectopic activation of the Notch signaling pathway. Using a temperature sensitive allele of ed, I show that Ed is required to suppress sensory bristles and for proper wing vein specification during adult development. In these processes also, ed acts in close concert with genes of the Notch signaling pathway. Overexpression of the membrane-tethered extracellular region of Ed results in a dominant-negative phenotype. This phenotype is suppressed by overexpression of Enhancer of split m7 {E(spl)}and enhanced by overexpression of Delta. Thus Ed interacts synergistically with the Notch signaling pathway.
I have identified a paralog of Ed, Friend-of-echinoid (Fred). fred function was examined in transgenic flies using inducible RNAi. Suppression of fred in the developing wing disc results in specification of ectopic SOPs, additional microchaeta and cell death. These phenotypes can be suppressed by increasing the activity of the Notch signaling pathway. Dosage-sensitive genetic interaction suggests a close functional relationship between fred and ed.
Microarray analysis of fred RNAi discs revealed a number of genes that are misregulated in the absence of Fred activity. Changes in the transcript levels predicted by the microarray data were validated by performing in situ hybridizations. One of the genes that is downregulated is pannier (pnr). Genetic interaction experiments suggest that Pnr cooperates with Fred to suppress neurogenesis in the apparently non-neurogenic regions of the wing disc.